Use SyncMessageFilter to Send() ipc messages from background threads.

content/common/web_database_observer_impl.cc

-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/common/web_database_observer_impl.h"
 
 #include "base/metrics/histogram.h"
 #include "base/string16.h"
 #include "content/common/database_messages.h"
 #include "third_party/sqlite/sqlite3.h"
 #include "third_party/WebKit/Source/WebKit/chromium/public/WebDatabase.h"
@@ skipping 42 matching lines @@
       if (result) { \
         UMA_HISTOGRAM_ENUMERATION("websql.Async." name ".ErrorSite", \
                                   callsite, kCallsiteHistogramSize); \
       } \
     } \
   } while (0)
 
 }  // namespace
 
 WebDatabaseObserverImpl::WebDatabaseObserverImpl(
-    IPC::Message::Sender* sender)
+    IPC::SyncMessageFilter* sender)
     : sender_(sender),
       open_connections_(new webkit_database::DatabaseConnectionsWrapper) {
+  DCHECK(sender);
 }
 
 WebDatabaseObserverImpl::~WebDatabaseObserverImpl() {
 }
 
 void WebDatabaseObserverImpl::databaseOpened(
     const WebDatabase& database) {
   string16 origin_identifier = database.securityOrigin().databaseIdentifier();
   string16 database_name = database.name();
   open_connections_->AddOpenConnection(origin_identifier, database_name);
@@ skipping 79 matching lines @@
   // We filter out errors which the backend doesn't act on to avoid
   // a unnecessary ipc traffic, this method can get called at a fairly
   // high frequency (per-sqlstatement).
   if (error == SQLITE_CORRUPT || error == SQLITE_NOTADB) {
     sender_->Send(new DatabaseHostMsg_HandleSqliteError(
         database.securityOrigin().databaseIdentifier(),
         database.name(),
         error));
   }
 }